Combined Effects of Ocean Warming and Acidification on Copepod Abundance, Body Size and Fatty Acid Content

Abstract

Concerns about increasing atmospheric CO2 concentrations and global warming have initiated studies on the consequences of multiple-stressor interactions on marine organisms and ecosystems. We present a fully-crossed factorial mesocosm study and assess how warming and acidification affect the abundance, body size, and fatty acid composition of copepods as a measure of nutritional quality. The experimental set-up allowed us to determine whether the effects of warming and acidification act additively, synergistically, or antagonistically on the abundance, body size, and fatty acid content of copepods, a major group of lower level consumers in marine food webs. Copepodite (developmental stages 1-5) and nauplii abundance were antagonistically affected by warming and acidification. Higher temperature decreased copepodite and nauplii abundance, while acidification partially compensated for the temperature effect. The abundance of adult copepods was negatively affected by warming. The prosome length of copepods was significantly reduced by warming, and the interaction of warming and CO2 antagonistically affected prosome length. Fatty acid composition was also significantly affected by warming. The content of saturated fatty acids increased, and the ratios of the polyunsaturated essential fatty acids docosahexaenoic- (DHA) and arachidonic acid (ARA) to total fatty acid content increased with higher temperatures. Additionally, here was a significant additive interaction effect of both parameters on arachidonic acid. Our results indicate that in a future ocean scenario, acidification might partially counteract some observed effects of increased temperature on zooplankton, while adding to others. These may be results of a fertilizing effect on phytoplankton as a copepod food source. In summary, copepod populations will be more strongly affected by warming rather than by acidifying oceans, but ocean acidification effects can modify some temperature impacts.

Fig 1

Temporal development of (A) mean pCO₂ and (B) mean dissolved inorganic carbon of each treatment. Error bars denote ± 1 SE (n = 3). Open symbols represent high pCO₂ (1400 μatm) and closes symbols low pCO₂ (560 μatm) concentrations. Symbols for the treatment combinations as in key.

Fig 2. Mean abundance of nauplii, copepodites and adults of the last experimental day.

Error bars denote ± 1 SE (n = 3). Open symbols represent high pCO₂ (1400 μatm) and closes symbols low pCO₂ (560 μatm) concentrations. Symbols for the treatment combinations as in key.

Fig 3. The nature (synergistic, additive, antagonistic) and magnitude of the observed interactions of temperature and OA on copepods.

Error bars denote ± 1 SE (n = 3).

Fig 4. Mean copepod taxon distribution for all treatments of the last experimental day.

Symbols for the taxon as in key.

Fig 5

Mean prosome lengths in μm of the last experimental day of (A) all occurring adults of each taxon and (B) all developmental stages of Paracalanus sp. Open symbols represent high pCO₂ (1400 μatm) and closes symbols low pCO₂ (560 μatm) concentrations. Symbols for the treatment combinations as in key. Error bars denote ± 1 SE (n = 3).

Fig 6. Mean fatty acid composition of one adult Paracalanus sp. of the last experimental day.

(A) total fatty acid (TFA) content, (B) saturated FA / TFA, (C) polyunsaturated FA / TFA, (D) DHA / TFA, (E) EPA / TFA, (F) ARA / TFA (raw FA data S5 Table). Error bars denote for ± 1 SD (n = 3).